Thyroid Hormone Mediation in Skull Development of Siberian Newt, Salamandrella keyserlingi (Urodela: Hynobiidae), with Comparison to Other Species

Abstract

Sequence and timing of cranial bones appearance and metamorphic transformation were compared in larvae of Siberian newt, Salamandrella keyserlingi (Urodela, Hynobiidae), reared under different levels of thyroid hormone (TH): i) normal, ii) high (in 1 and 10 ng/ml T 3 ), and iii) low (in 0.02% thiourea, TU, which inhibits thyroid gland activity resulting in TH deficiency). Cranial bones exhibit differential reaction to changes in TH level. Bones appearing in early larval ontogeny display no or only slight response to TH changes. At different TH regimes, they arise at the same stages and at the same age. Bones appearing later show reaction to TH changes: their appearance is accelerated and retarded in TH- and TU-treated larvae, correspondingly. The major response to TH alterations is displayed by ontogenetic events involved into metamorphic remodeling: they are greatly accelerated under TH-treatment and retarded in TH-deficiency. Most metamorphic events fail to occur under TH-deficiency. Comparison of TH-related changes in skull development of Ambystoma mexicanum ( Ambystoma tidae), Triturus vulgaris (Salamandridae), and S. keyserlingi showed that in these species the same bones differ in reaction to TH changes. A corollary of these differences is that in urodeles TH-dependence and TH-sensitivity of cranial bones change both in ontogeny and phylogeny.